The barometer has been called a weather glass. Rules are attempted to be established, by which, from the height of the mercury, the coming state of the weather may be predicted, and we accordingly find the words “Rain,” “Fair,” “Changeable,” “Frost,” &c., engraved on the scale attached to common domestic barometers, as if, when the mercury stands at the height marked by these words, the weather is always subject to the vicissitudes expressed by them. These marks are, however, entitled to no attention; and it is only surprising to find their use continued in the present times, when knowledge is so widely diffused. They are, in fact, to be ranked scarcely above the vox stellarum, or astrological almanac.
Two barometers, one near the level of the River Thames, and the other on the heights of Hampstead, will differ by half an inch; the latter being always half an inch lower than the former. If the words, therefore, engraved upon the plates are to be relied on, similar changes of weather could never happen at these two situations. But what is even more absurd, such a scale would inform us that the weather at the foot of a high building, such as St. Paul’s, must always be different from the weather at the top of it.
It is observed that the changes of weather are indicated, not by the actual height of the mercury, but by its change of height. One of the most general, though not absolutely invariable, rules is, that when the mercury is very low, and therefore the atmosphere very light, high winds and storms may be expected.
The following rules may generally be relied upon, at least to a certain extent:
- Generally the rising of the mercury indicates the approach of fair weather; the falling of it shows the approach of foul weather.
- In sultry weather the fall of the mercury indicates coming thunder. In winter, the rise of the mercury indicates frost. In frost, its fall indicates thaw; and its rise indicates snow.
- Whatever change of weather suddenly follows a change in the barometer, may be expected to last but a short time. Thus, if fair weather follow immediately the rise of the mercury, there will be very little of it; and, in the same way if foul weather follow the fall of the mercury, it will last but a short time.
- If fair weather continue for several days, during which the mercury continually falls, a long continuance of foul weather will probably ensue; and again, if foul weather continue for several days, while the mercury continually rises, it long succession of fair weather will probably succeed.
- A fluctuating and unsettled state in the mercurial column indicates changeable weather.
The domestic barometer would become a much more useful instrument, if, instead of the words usually engraved on the plate, a short list of the best established rules, such as the above, accompanied it, which might be either engraved on the plate, or printed on a card. It would be right, however, to express the rules only with that degree of probability which observation of past phenomena has justified. There is no rule respecting these effects which will hold good with perfect certainty in every case.
This volume, we should add, is by Dr. Lardner, the editor of the Cyclopædia, and is a good model for his collaborateurs.
REFLECTION.
It is better to reflect ourselves, than to suffer others to reflect for us. A philosopher has a system; he views things according to his theory; he is unavoidably partial; and, like Lucian’s painter, he paints his one-eyed princes in profile.